This application is for continuation of support of studies in progress aimed at testing the hypothesis of a Ca2 ion-induced release of Ca2 ion from the sarcoplasmic reticulum as a step in the physiological excitation-contraction coupling in mammalian cardiac muscle. The methods will use a preparation of skinned cardiac cells (sarcolemma removed by micro-dissection) that has been developed by the applicants, tension recording from these skinned cardiac cells, recording of the potential across the sarcoplamsic reticulum membrane in skinned cells stained with potential sensitive dyes, measurements of the Ca2 ion binding by the sarcoplasmic reticulum using the fluorescence of chlorotetracycline, and, hopefully, direct detection of the changes of myoplasmce (free Ca2 ion) concentration using dual-wavelength spectrophotometry with arsenazo III and Ca2 ion sensitive microelectrodes. The new projects of research will be: 1) An evaluation of how Ca2 ion release of Ca2 ion from the sarcoplasmic reticulum may be integrated in among the other steps of cardiac excitation-contraction coupling (experimental approach and modeling). 2) A study of the effects of the monovalent cations K ion, Na ion, and Li ion, the divalent cations Ba2 ion and Sr2 ion and of anions variably premeant on the process of Ca2 ion release from and accumulation by the sarcoplasmic reticulum. The aims are to attempt to define the mechanics of the Ca2 ion-induced release of Ca 2 ion from the sarcoplasmic reticulum by testing whether a translocation of counter-ions occurs during Ca2 ion release and accumulation and to possibly identify possible alternative or additional hypotheses for the mechanism of Ca 2 ion release from the sarcoplasmic reticulum (e.g., Na ion-induced or "depolarization-induced" release of Ca2 ion). Another hypothesis to be tested for the mechanism of the Ca2 ion-induced release of Ca2 ion from the sarcoplasmic reticulum will be a possible link between this process and the process of Ca2 ion accumulation by the sarcoplasmic reticulum.